This invention relates to a pulsed brushless direct current (dc) motor and particularly to such a motor including an electronic control for energizing of the motor to maintain motor operation in the presence of varying motor torques, available line voltages and the like.
Brushless dc motors have been developed for precise precision drives in various applications. Brushless dc motors, for example, are now widely used in disk drives for computing systems. In such motors, a rotor includes fixed polarity poles. The stator winding includes a plurality of spaced windings. Energizing of the windings in sequence with a dc signal generates an interacting field with that of the rotor poles to establish rotation of the rotor. The pulsing of the stator windings at predetermined orientations of the rotor with respect to the stator windings is necessary to provide maximum torque per power pulse application of the windings, and is critical in order to maintain and establish a desired speed characteristic with varying torque, voltages and the like. A widely used system includes Hall cell sensors mounted to sense the position of the rotor relative to the stator. The output of the Hall cell produces an appropriate signal to the control system for switching power from one winding to another at the proper commutation time for the windings. The Hall cell elements of course contribute to the cost of the motor, which is further increased because of the necessity to provide precise and accurate mounting of the sensors as well as the additional leads from the motor proper to the electronic control system. Generally, the system requires a specially shielded lead, and each motor is more or less constructed as a custom design and does not lend itself to building of a single standard motor line. Hall cell elements are semi-conductor based structures, therefore have temperature limitations which must be considered, and inherently raise a problem with respect to reliability of the motor operation. Motor derating may be specified to anticipate adverse temperature conditions.
An alternative sensing system which has been suggested is based on an electronic sensing system including directly sensing the electrical characteristic in an unenergized winding of the stator for detecting the desired time for commutating the motor windings. U.S. Pat. No. 4,027,215 which issued May 31, 1977 discloses a zero crossing detection system for brushless dc motors to generate commutating signals. That patent also discusses the background of dc and A.C. machines and further discusses some of the disadvantages of the brushless dc motors which rely on separate sensors for detecting rotor position and thereby the appropriate commutation times. The '215 patent provides a two stage counter controlled by the output of a signal induced in the unenergized winding for purposes of detecting a proper commutation time. Thus, in a three phase stator winding, the three power coils are equicircumferentially spaced or wound on the stator. Each winding is pulsed during the period when the other two windings would generate a signal less than the signal of the selected winding. Thus, assuming a three phase time sequence with a sine wave characteristic, each winding would be energized during its positive half cycle and particularly between the period between 30.degree. and 150.degree. of such half cycle, or for the 120.degree. of each cycle of rotation, with successive windings being energized sequentially to energize only two windings simultaneously. The system used therein detects the zero crossing and then activates a pair of counters for each phase to monitor the zero crossings and the particular time of commutation. Actual commutation is created by a plurality of solid state switches interconnecting of the motor winding dc supply. The electronic control system as disclosed in that patent provides a high degree of complexity to avoid the necessity of the separate position sensing elements. The use of the up-down counters provides for counting of the unit at one rate, such as counting up at one rate during the zero crossing and then counting downwardly at such rate to generate a pulse at a particular desired delay time period and then recycling of the system to maintain the continued operation of the two stage counter system. Until the motor reaches a certain selected speed of operation, the unenergized winding does not provide an appropriate signal for monitoring and controlling commutation. The motor is therefore started with a time spaced pulsed control with sufficient energy supply to initiate the operation of the motor as a more or less conventional stepping motor wherein the windings are sequentially energized to start the motor and accelerate the motor to a speed wherein an effective detectable voltage appears in the unenergized winding to permit the continued for operation with the zero crossing detection. Other controls are also disclosed in the patent to provide for varying of the speed and centering of the pulses to maintain maximum efficiency and the like.